When color images are formed by utilizing ink compositions, at least yellow ink, magenta ink and cyan ink are generally used. And it is possible to reproduce colors in the green region when yellow and cyan colors are mixed, colors in the red region when yellow and magenta colors are mixed, and colors in the blue region when cyan and magenta colors are mixed. On this account, it has been long desired to develop an ink set which can excellently reproduce a wide range of colors in each of those regions.
It is necessary for each ink to meet general performance requirements for an ink composition. For instance, it is required for an ink composition to form clear images on a recording material and, during long-term storage, not to cause degradation in the images formed. And light resistance in particular is given as one of performance requirements for images formed with ink compositions.
For the purpose of imparting satisfactory light resistance to ink compositions, pigments are generally used as coloring agents in the ink compositions. However, there are cases where, depending on the types chosen, pigments cannot impart satisfactory light resistance to ink compositions. In addition, pigments are inferior to dyes in tinctorial strength, and besides, it happens that images obtained by printing on a recording material are lacking in vividness.
Moreover, a color image formed with a plurality of ink compositions, even if one of colors present therein is inferior in light resistance, suffers a change in hue, and thereby the quality of the color image is degraded in the extreme. Consequently, color ink compositions are required to have more controlled light resistance.
On the other hand, recently have printers for inkjet recording come into widespread use. A method of this inkjet recording is a printing method that liquid droplets of ink compositions are made to fly out and adhere to a recording material, such as paper, and thereby printing is performed. This method features the feasibility of printing high-resolution and high-quality images at a high speed by means of moderately priced apparatus. Where color inkjet recording apparatus in particular is concerned, images produced therein have moved up in quality. As a result, the apparatus is also used as an output device of photographs, and further has reached a level that allows even utilizations as a digital printer, a plotter, a CAD output device and the like. It is conceivable that images printed with a printer for inkjet recording may be used in a wide variety of forms. It is imaginable that the printed matter of photograph specifications or the like, in particular, will be kept for a long time as a display on an indoor place exposed to a fluorescent lamp or on an outdoor place or a like place exposed to direct sunlight. The light resistance is therefore very important performance required of images obtained by the method of inkjet recording.
As to the pigment ink set having a wide range of color reproducibility and excellent light resistance, yellow pigment ink satisfying performance requirements at high levels, notably one which allows compatibility between good hue and high-level light resistance, is not yet in existence, and development of such yellow pigment ink has been long desired.
In forming color images, a yellow ink composition containing a yellow pigment is used for reproduction of a color in the yellow region. Examples of a yellow pigment include C.I. Pigment Yellows 1, 2, 3, 12, 13, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 128, 129, 138, 139, 150, 151, 154, 155, 180, 185, 213 and so on.
Furthermore, it is disclosed in Patent Documents 1 and 2 that reproduction of a wide range of colors in the yellow region can be improved by preparing a plurality of yellow pigment compositions into each of which some of the above-cited yellow pigments are incorporated in combination.
However, yellow ink compositions each containing two or more of yellow pigments having satisfactory color reproducibility are not sufficient in dark color reproducibility in the yellow region though they can attain color reproducibility over a wider range than ink containing one kind of pigment. In addition, when pigments apart from each other in hue angle are mixed, there is a remarkable tendency of secondary color saturation to be lowered. For instance, when C.I. Pigment yellow 110 is used in an ink set, the ink set has a drawback of being low in green color saturation because the pigment is tinged with red color.
Further, such a yellow ink composition as not only to provide clear images on a recording material but also to cause no degradation in the images during long-term storage (to have light resistance in particular) is desired. When a color image is formed by use of a plurality of pigment ink compositions, the color image suffers a change in hue so long as any one of the pigments present therein is inferior in light resistance, and the quality thereof is extremely degraded. For instance, C.I. Pigment Yellow 74 is markedly low in light resistance though relatively good in hue, and when it is used in an ink set, images in yellow, red, green and gray portions suffer substantial changes in individual hues. Thus quality degradation of color images has been becoming a large problem.
For the purpose of reproducing a wide range of colors in the yellow region, on the other hand, methods of using ink sets which each, aside from yellow ink having high lightness in color, further includes a second yellow ink composition referred to as dark yellow, namely yellow ink having low lightness in color, are disclosed in Patent Documents 3, 4, 5 and 6.
However, methods of using second ink compositions, typified by dark yellow, bring about an increase in the number of ink colors, and thereby color making becomes complicated. As a consequence, not only such methods have developed a tendency to raise the cost but also improvements produced thereby have not yet reached such a level as to sufficiently satisfy performance requirements.